Contemporary hard disk drives include an actuator assembly pivoting through an actuator pivot to position one or more read-write heads, embedded in sliders, each over a disk surface. The data stored on the disk surface is typically arranged in concentric tracks. To access the data of a track, a servo controller first positions the read-write head by electrically stimulating the voice coil motor, which couples through the voice coil and an actuator arm to move a head gimbal assembly in lateral positioning the slider close to the track. Once the read-write head is close to the track, the servo controller typically enters an operational mode known herein as track following. It is during track following mode that the read-write head is used to access the data stored on the track.
Micro-actuators provide a second actuation stage for lateral positioning the read-write head during track following mode. They often use an electrostatic effect and/or a piezoelectric effect to rapidly make fine position changes. They have doubled the bandwidth of servo controllers and are believed essential for high capacity hard disk drives from hereon. To minimize Repeatable Run-Out errors, a correction function is written onto the disk surface and used to correct the repeatable errors. With the development of micro-actuators, servo controllers have had a need to support both single stage actuation using just the voice coil motor, and dual stage actuation also using the micro-actuator. However, there are several problems the inventors have encountered during their work with controlling the positioning of sliders above a disk surface in hard disk drives. Straightforward approaches to these correction functions leads to storing two corrector functions for each track, doubling the storage overhead for each track on the disk surface. One of the corrector functions works with the single stage actuator mode and the other works with the dual stage actuator mode of the servo controller. There is another, trickier problem, having to do with the possibility of injecting discontinuities caused by the differences between these corrector functions when the servo controller switches between single and dual stage actuation.
What is needed are corrector functions for Repeatable Run-Out which require nearly the same storage overhead as functions used in single stage actuator hard disk drives, which seamlessly support the servo controller in single stage and multi-stage actuator modes. What is needed are hard disk drives storing these functions for tracks on their disk surfaces incorporating these advantages.